Using GaN HEMTs for high performance in USB-C designs
GaN HEMTs operate at the high switching frequencies need for high power density USB-C adapter and charger designs. Luo Junyang, technical marketing at Infineon Technologies, discusses alternative approaches to implementing common topologies found in USB-C power delivery (PD) designs.
Modern USB-PD adapters and chargers (including those with extended power range) are required to deliver exceptional performance in the smallest possible form factors. There are a range of topologies commonly used: active clamp flyback, hybrid flyback, as well as PFC and hybrid for extended power range. But what are the pros and cons of implementing them using discrete devices versus fully integrated GaN HEMTs? And what might be the benefits from using integrated switching controllers, where appropriate to a topology?
Active clamp flyback - Discrete
We’ll start with one of the most popular topologies: the active clamp flyback (ACF) converter which enables zero voltage switching (ZVS) for power switches. The topology (Figure 1) is suitable for higher frequency operation than a quasi-resonant flyback due to the ZVS operation and complete recovery of the energy in transformer leakage inductance dissipated in the flyback resistor-capacitor-diode (RCD) clamp. Clamping the primary-switch voltage through the clamp capacitor (combined with ZVS) means the ACF topology provides good electromagnetic interference (EMI) performance.
Figure 1 Discrete implementation of the Active Flyback Converter Topology
Two ACF control schemes exist: complementary (CP) and non-complementary (NCP). In CP mode, the main flyback switch and clamp switch turn on and off in a complementary fashion in each switching cycle, so the transformer current waveform has an approximately sinusoidal shape. The main flyback switch and clamp switch operate under ZVS switching, with the switching frequency increasing as the load decreases. This can impact light-load and standby efficiency as well as affect EMI.
Additionally, the total resonant current in the resonant tank causes conduction losses in the transformer to increase. NCP ACF can be used to overcome both of these issues. In this mode, the active clamp switch only turns on sufficient magnetic energy to maintain ZVS in the main switch. This way, the circulating clamp capacitor currents are mitigated.
Active clamp flyback - Integrated
Figure 2 shows an alternative implementation of the active clamp flyback (ACF) using a CoolGaN integrated power stage (IPS). Here the clamp switch provides a path to recover the energy stored in the transformer’s leakage inductance (Llk) when the main switch turns off, and the clamp switch turns on. As a result, Cclmp and Llk resonate together through the clamp switch and the transformer, resulting in energy transfer to the load. This energy recovery increases the system efficiency compared to the passive clamp flyback, where the energy is stored in Llk damps in a traditional RCD clamp circuit.
Figure 2 Active Flyback Converter Topology implemented using IPS
Hybrid flyback – Discrete
The hybrid flyback (HFB) converter (Figure 3) is another resonant topology that not only employs ZVS but zero current switching (ZCS) too. The primary-side converter has resonant-type current waveforms, which means high-frequency, high efficiency is possible due to ZVS operation with lower RMS currents. In addition, the capacity of the resonant capacitor assists with energy storage, which means the transformer size can be smaller than for other flyback-type topologies.
Thanks to the half-bridge structure with a self-voltage clamp to Vbus on the primary side, the voltage stress on the switching device is better than for the ACF topology. HFB has an additional switch on the primary side compared to QR flyback, which requires special care for the light-load efficiency due to the circulating currents of the resonant type operation. Careful design is needed for universal input specifications, and the control complexity required for HFB is much higher than that needed for the QR flyback topology. However, Infineon’s XDP digital power XDPS2201 controller uses a dedicated control algorithm, greatly simplifying this task.
Figure 3 Discrete implementation of the Hybrid Flyback Topology with XDPS2201 controller
Hybrid flyback – Integrated
Figure 4 shows a hybrid flyback converter (HFB) topology with the CoolGaN IPS. The converter consists of a high-side and a low-side switch, the transformer, the resonant tank (Llk and Cr), the output stage rectifier and capacitors. An advanced control scheme with a non-complimentary switching pattern provides a solution that supports a wide range of AC input and DC output voltages, which is necessary for universal USB-C PD operation.
When the high-side switch is turned on, the hybrid flyback converter stores energy in the primary-side inductor. The energy stored in the primary side inductor is transferred to the output when the low-side switch is turned on. With proper timing control during the switch transition of both switches, HFB runs under ZVS for both devices, ensuring high system efficiency without requiring additional components. Both devices benefit from ZVS and higher efficiency (from ZCS operation on the secondary side), making hybrid flyback a cost-competitive solution for ultra-high power density converters, like USB-PD fast chargers.
